SEVEN out of 10 prisoners are taking heroin, as drug testing regimes inadvertently turn inmates into addicts, it is claimed today.
One ex-con from Cardiff Prison claims that a drug testing regime introduced nine years ago has made the drug problem worse.
The man, from West Wales, claims that Mandatory Drug Testing (MDT) is driving convicts away from cannabis and towards heroin.
He was originally imprisoned for nine months for various driving offences.
When he went in he'd only ever taken cannabis, but when he came out he was hooked on heroin.
He speaks out on tonight's S4C current affairs programme Y Byd ar Bedwar.
"Cardiff Prison made me a smack-head - a heroin addict," he tells the programme.
"And I don't think I would be in this position if I hadn't been there."
He had already been to prison once before in the late '80s and says that the introduction of MDT changed everything. When he was sent to Cardiff Prison five years ago, he claims the atmosphere was far more tense.
"It was like comparing a holiday in Tenby to a holiday in Beirut.
"The only place I've been where you can get it (heroin) 365 days a year is Cardiff Prison.
"In the nine months, I was probably offered cannabis three times at the most.
"I was offered heroin every single day I was in there."
For the majority of his sentence he says he didn't take drugs, but weeks before he was due for release, somebody very close to him died.
When he wanted something to help him cope, he claims the only option widely available in Cardiff Prison was heroin.
The MDT regime was introduced in 1996 as a deterrent to drug users in prison. But ex-inmates who spoke to the programme say prisoners were switching to heroin because they were less likely to get caught.
Cannabis can stay in your system for a month, whereas heroin usually leaves the body in a couple of days.
Clive Roberts was a well-known actor on Welsh television in the '80s, appearing on Fo a Fe and Pobol y Cwm.
He served 12 years in prison for the murder of his partner, and agrees that MDT caused a dramatic shift in the amount of heroin taken in prison.
"The truth is that 70% of prisoners are on heroin and the fact is that when I went to prison (in 1989) they weren't, 70% were on cannabis," he said.
"It may not be important to people what happens to prisoners, but they don't want 70% of them to come out on the streets as heroin addicts."
He says he did not take heroin, but quickly became aware of the techniques involved.
"The cons knew the rules and that was the problem.
"Because cannabis would stay in the system for a month, but heroin would be out of the system in two days, they would drink a copious amount of water if they were on heroin.
"Then if they had to take a Mandatory Drug Test, it would come back as 'dilute', and they knew they'd have another test in a week's time and could stay clean for that."
Cardiff Prison says it is working hard to combat a serious problem.
Deputy Governor Sean Sullivan accepts that the level of drug use at the prison is high, but he is adamant that the benefits of MDT outweigh any problems.
"I think the other side of that is that MDT is identifying individuals who are still using illicit substances while in prison, and that gives us an opportunity to start to help them to deal with those particular problems.
"I would be concerned about anybody who didn't have a drug problem before coming into custody who developed that problem in custody.
"I would certainly be concerned about anybody that was in that situation, but all I'm saying is that the research doesn't bear out that that is happening on a large scale."
One ex-con says he's still struggling with the heroin problem he says he inherited from his time in HMP Cardiff.
"I think the prison service do a great job in promoting it - they're the best drug dealers I've ever met, and that's not bitterness because I've been in prison, that's just the truth."
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